This invention relates to electromechanical assemblies of the type which include: a) an electrical component having a planar array of input/output pads, b) a substrate having a matching planar array of input/output pads, and c) respective solder joints that connect corresponding input/output pads of the electrical component and the substrate. More particularly, this invention relates to processes for stretching the solder joints in the above type of electromechanical assemblies in order to make the assemblies less susceptible to failure, due to the reduction of thermally induced stress and strain.
One specific example of the above type of assembly is where the electrical component is an integrated circuit die and the substrate is a cofired multi-layered ceramic substrate. To fabricate such an assembly, a respective "solder ball" is deposited on each of the input/output pads of the die. Typically, this is achieved by providing a mask which has an array of holes that only expose the die's input/output pads, and by vaporizing solder through the mask onto the input/output pads. Thereafter, the solder on the die is reflowed by a heating step, and then the die is placed on the substrate such that the die's solder balls are aligned with the substrate input/output pads. Then the solder balls are heated until they melt; and, lastly, the solder is cooled and resolidified to thereby form the solder joints between the aligned input/output pads.
During the above described prior art process, each of the solder joints is formed with a convex shape (i.e.--barrel shape). This shape occurs because when the solder balls are melted, the surface tension in each molten solder ball tends to minimize the solder ball's surface area; and for any given volume of molten solder, the minimum surface area is reached when the shape is spherical.
However, convex shaped solder joints have a major drawback when thermally induced stress and strain are considered. These stresses and strains arise when the die and the substrate are repeatedly heated and cooled during their operation. Due to that heating and cooling, the die and the substrate expand and contract; and if the die and the substrate have different thermal expansion coefficients, they will expand and contract by a different amount. In each solder joint, the average strain equals the difference by which the ends of the joint move divided by the height of the joint. Thus, as the joint shape bulges outward more, the joint height gets smaller for any given solder volume; and that decrease in joint height makes the strain larger. Likewise, the joint stress gets larger since it increases whenever strain increases.
A recent paper which discusses the above stress/strain problem is: "Mechanical Design Considerations For Area Array Solder Joints" by Peter Borgenson, Chi-Lu Li and H. D. Conway, IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Volume 16, No. 3, May 1993 at pages 272-283. In that paper, the conventional convex shaped solder joint is shown in FIG. 4 on page 275.
Following FIG. 4, the paper goes through a mathematical analysis which shows that the stress in a mathematically modeled solder joint can be reduced if the joint length is somehow increased until the shape of the joint changes from convex to concave. Then, to actually build a real concave shaped solder joint, the paper on page 277 says: "In principal, different solder joint shapes may be achieved by two types of means. For one, we may, for example, vary either solder volume or pad size. Alternatively, we may somehow apply an additional downward or upward force during reflow".
However, in the remainder of the paper, the only actual method which is disclosed for fabricating a concave shaped solder joint is where the solder volume is varied; and that method is described in conjunction with FIG. 18. According to that disclosed method, solder balls are deposited on the die interior with a large volume of solder, and solder balls are deposited on the die periphery with a small volume of solder. Since the interior solder balls have a large volume of solder, those interior solder balls push the die farther away from the substrate than the periphery solder balls. Consequently, the solder balls on the periphery of the die get stretched and thereby achieve a concave shape.
But one major drawback with the above described method is that as the amount of solder in each interior solder ball increases, those solder balls become so large that the small periphery solder balls do not contact the input/output pads on the substrate when all of the solder balls are melted. Consequently, an additional undisclosed step would be required to somehow squeeze the large interior solder balls while they are molten until each small periphery solder ball contacts and forms a joint with the substrate input/output pads. Also, while the large molten interior solder balls are squeezed, a second additional undisclosed step would be required to somehow insure that the die does not tip and thereby cause any molten solder ball to be squeezed so much that it "squirts" off of it's input/output pad. Further, after the small molten solder balls on the die periphery have contacted the substrate input/output pads, a third undisclosed additional step would be required wherein the squeezing force is somehow removed so the small periphery solder balls get stretched.
Another drawback of the above described process is that as the amount of solder in each interior solder ball is increased, the cross-sectional area of that solder ball must likewise be increased. That is because when the solder ball is initially deposited on the input/output pad by vaporizing solder through a mask, it is impossible to exceed a certain height-to-width aspect ratio for the solder ball. Consequently, if it is desired to double the height of the interior solder balls, then the radius of those solder balls must also be approximately doubled. However, doubling the radius of the internal solder balls means that their cross-sectional area will be increased approximately by a factor of four; and thus the density with which an array of those solder balls can be formed will be decreased by approximately a factor of four. This is a serious problem because it limits the number of input/output signals that can be sent to/received from a die of any given size.
Accordingly, a primary object of the invention is to provide an improved method of elongating the solder joints in an electromechanical assembly wherein the above problems are overcome.